In one known method of making such an assembly the sealing ring portion and insert are produced separately by moulding or extrusion. The insert is then fitted into the channel or cavity of the sealing ring portion where it is retained mechanically. However this method does not generally produce a very good match between channel and insert because small differences and inconsistencies in size and/or shape of the two parts tend to arise in manufacture. As a result there is a variation in contact and surface reaction over the opposing insert and channel surfaces in the assembly, and this in turn causes undesirable variation in effectiveness of seals as they age and/or the pressure on them is increased.
A second known method which in part avoids the problems described above involves moulding the insert using the channel of the sealing ring portion. The sealing ring portion is preformed with a channel of the desired shape by injection moulding, and then itself used as part of a mould in which the insert is subsequently formed. Such a method does however have certain drawbacks. The insert is necessarily moulded and not extruded which may be uneconomical in certain cases. More importantly, chemical bonding is needed between the two parts to hold the insert in position. With this method it is not generally possible to use the most effective insert materials with very low compression set because the high temperature thermosetting conditions for these would damage the thermoplastics sealing ring portion in which they were moulded.
In order to achieve the required chemical bonding it may be necessary to add an extra step to the production process. Because the surface to be treated is a concave channel surface, this extra step tends to be inconvenient, costly and a substantial disincentive to use of this method despite the other advantages of the method.
It is thus desirable to produce assemblies with a low compression set insert and a good and consistent fit between the parts, without having to carry out an inconvenient bonding step.